1. Field of the Invention
The present invention relates to a liquid ejection apparatus and an electric field application method, and more particularly to a technique of forming images and the like in a liquid ejection apparatus using an electrorheological liquid.
2. Description of the Related Art
In recent years, inkjet recording apparatuses have come into wide use as data output apparatuses for outputting images, documents, and the like. An inkjet recording apparatus is capable of forming an image (data) on a medium such as recording paper by depositing ink by driving nozzles provided in a print head in accordance with data so that the ink is ejected from the nozzles.
Demands for improved printed image quality and productivity are being made in relation to inkjet recording apparatuses. To realize higher printed image quality, the dots constituting the image must be made as small as possible, and these minute dots must be disposed at a high density. To improve productivity, the moving speed of the print head or the conveyance speed of the medium must be increased, and the scanning system, conveyance system, and ejection timing must be controlled accordingly to reduce the length of the ejection cycle.
However, when dots are disposed at high density, adjacent or surrounding dots overlap, and when ink for forming such dots is ejected in a short ejection cycle, an ink droplet lands on the medium while the ink droplet that has previously landed on the medium and not yet become fixed. As a result, the ink droplet that lands initially and the ink droplet that lands subsequently interfere with each other (landing interference), causing problems such as uneven coloring and line width variation.
In general, bleeding, dot spread, color mixing, and so on occur when using the low-viscosity ink, and these phenomena lead to great deterioration in the quality of the printed image.
There are proposed methods for suppressing deterioration in the quality of the printed image caused by landing interference, bleeding, and so on in an inkjet recording apparatus, in which an image is formed on the medium using an electrorheological ink that exhibits an electrorheological effect in which the viscosity of the ink is increased through application of an electric field. The electrorheological inks can be broadly classified into a dispersion type and a uniform type. In the dispersion type, dielectric particles are dispersed through the liquid, the particles are polarized by an electric field such that the polarized particles form bridges or chain-form clusters along the electric field direction, and these bridges and so on increase the viscosity of the ink. In the uniform type, the molecules or domains in the liquid are oriented along the electric field direction when an electric field is applied, thereby exhibiting anisotropy.
Japanese Patent Application Publication No. 2-212149 discloses an image forming method, in which an electric field is applied to a recording body possessing the electrorheological effect, thereby suppressing penetration of the recording body so that blurring and concentration deterioration are prevented.
Japanese Patent Application Publication No. 5-4342 discloses a recording apparatus, in which a recording head deposits a recording liquid having an electrorheological effect onto an intermediate transfer medium on which an electric field is formed, thereby increasing the viscosity of the recording liquid on the intermediate transfer medium so that excessive dot spread and color mixing can be prevented. When the viscosity of the recording liquid has increased through drying or the viscosity of the recording liquid has increased through the electrorheological effect thereof, the recording liquid is transferred from the intermediate transfer medium to a transfer subject medium.
Japanese Patent Application Publication No. 5-4343 discloses a recording apparatus, in which a recording head deposits a recording liquid droplet possessing the electrorheological effect onto a transfer subject medium to which an electric field is applied, thereby increasing the viscosity of the recording liquid on the transfer subject medium so that the dots formed by the recording liquid do not suffer from spread, bleeding, and color mixing. The electric field is maintained while drying of the recording liquid and penetration into the transfer subject medium progress until bleeding and color mixing no longer occur.
The electrorheological ink has a property whereby the increase in ink viscosity varies proportionally with the direction and intensity of the applied electric field. In other words, the ink exhibits reversibility such that when an electric field is applied, the viscosity of the ink appears to instantaneously increase; however, when the electric field is cut off, the viscosity returns to its original level. Therefore, the electric field generation timing and generation time (period) should be controlled in accordance with ink ejection.
In the image forming method described in Japanese Patent Application Publication No. 2-212149, a recording member on which the recording body has been deposited is passed under an electric field generated by an electrode or corona charging device. With this constitution, it is difficult to generate an electric field that is capable of producing the electrorheological effect in the recording body.
In the recording apparatuses described in Japanese Patent Application Publication Nos. 5-4342 and 5-4343, a method of charging the intermediate transfer medium or recording subject medium prior to recording is employed; however, with this method, the intensity of the electric field is weakened by atmospheric discharge and the like so that a sufficient electric field for producing the electrorheological effect in the recording liquid cannot be obtained. Moreover, unless the electric field applied to the recording liquid is maintained until the recording liquid is fixed on the medium, landing interference, bleeding of the recording liquid, and dot spread cannot be suppressed.